Pure, node-free, headlessly testable. Carries a seed, not a stat block. Owns no
rules of its own: errors() delegates to NewGame.validate.
The interaction rules that make creation screens buggy are pinned by test:
changing calling clears the picks; changing race drops a pick the new race now
grants (the elf/cutpurse case); re-roll clears the spend and only the spend;
minus floors at the roll; the fourth point does not exist.
